When the biplane is completed it is to be fastened securely to the spar in such a position that it is accurately balanced--from side to side. The spar may be laid on a table, and the biplane placed across it in its approximate position. Then move the plane to one side until it tips down, and mark the spot on the rear edge of the plane. Repeat this operation toward the other side, and the centre between the two marks should be accurately fastened over the centre line of the spar. Even with the greatest care there may still be failure to balance exactly, but a little work with a file on the heavy side, or a bit of chewing gum stuck on the lighter side, will remedy the matter. The body of the aeroplane being now built, it is in order to fit it with propelling mechanism. The motive power to whirl the propeller we have already prepared is to be the torsion, or twisting strain--in this case the force of untwisting--of india rubber. When several strands of pure rubber cord are twisted up tight, their elasticity tends to untwist them with considerable force. The attachment for the rubber strands at the front end of the spar is a sort of bracket made of the brass wire. The ends of the wire are turned up just a little, and they are set into little holes in the under side of the spar. Where the wire turns downward to form the hook it is bound tightly to the spar with silk thread. The hook-shaped tip is formed of the loop of the wire doubled upon itself. The rear attachment of the rubber strands is a loop upon the propeller shaft itself. As shown in the drawings, this shaft is but a piece of the brass wire. On one end (the rear) an open loop is formed, and into this is slipped the centre of the propeller. The short end of the loop is then twisted around the longer shank--very carefully, lest the wire cut into and destroy the propeller. Two turns of the wire is enough, and then the tip of the twisted end should be worked down flat with the file, to serve as a bearing for the propeller against the thrust-block. This latter is made of a piece of sheet brass (a bit of printers’ brass “rule” is just the thing) about 1/40 of an inch thick. It should be ¼ of an inch wide except at the forward end, where it is to be filed to a long point and bent up a trifle to enter the wood of the spar. The rear end is bent down (not too sharply, lest it break) to form the bearing for the propeller, a hole being drilled through it for the propeller shaft, just large enough for the shaft to turn freely in it. Another smaller hole is to be drilled for a little screw to enter the rear end of the spar. Next pass the straight end of the propeller shaft through the hole drilled for it, and with the pliers form a round hook for the rear attachment of the rubber strands. Screw the brass bearing into place, and for additional strength, wind a binding of silk thread around it and the spar.